The present invention relates to a safety circuit for use in connection with an electrically heated blanket or bed cover. More particularly, it relates to a circuit intended for use in a blanket of the type using a positive temperature coefficient material as the heating element. It is an improvement over the circuit disclosed and claimed in U.S. patent application Ser. No. 136,202, filed Apr. 1, 1980.
Electric blankets are typically formed with fabric shells which include passageways throughout the area of the blanket in which a tortuous low wattage heating element is threaded. The blanket must be provided with some means for sensing overheat conditions along the heating element within the blanket so that the current to the blanket can be shut off or reduced before damage or injury is caused by the overheat condition. The various means for sensing such overheat conditions have included discrete bimetallic thermostats positioned at spaced intervals along the blanket. In addition, continuous sensing wires have been used in conjunction with the heating element wire. The sensing wire responds to overheat conditions to operate a relay which opens the circuit to the main heating element.
More recently, there has been consideration of the use of positive temperature coefficient materials for the heating element so as to provide a blanket wire which would be self-limiting from a temperature standpoint in any areas in which an overheat condition occurred. The blanket wire consists of two spaced conductors which are enclosed by a positive temperature coefficient material comprising polyethylene with carbon black particles mixed therewith. The electrical current passes through the positive temperature coefficient material in passing from the one conductor to the other conductor and the PTC material acts as a heating element.
The formulation of the PTC material and the physical dimensions of its extrusion is selected so that the resistance, and, therefore, the heat dissipation per foot of length are reasonably constant at any given temperature. At low temperatures, the heat dissipation per foot will be greater than at normal room temperatures. When in an overheat or high temperature condition, the heat dissipation will be less than normal. The PTC material self limits to produce a given heat dissipation or wire temperature for every different ambient and insulation system. In this way, when a section of the heater is bunched up or abnormally restricted insofar as heat transfer is concerned (something on top of the blanket), the PTC wire reacts to the new environment and reduces its heat dissipation in that area, trying to keep its temperature reasonably constant. A suitable PTC heating wire for use in connection with the present invention is disclosed in the U.S. patent to Kelly, U.S. Pat. No. 4,277,673.
Under normal circumstances, the type of PTC blanket wire described above operates well and eliminates the necessity for either the discrete bimetallic thermostats within the blanket or the various types of distributed sensing wires paralleling the heater wires in the blanket. However, it has been ascertained that significant problems arise when a broken or open circuit occurs in connection with one of the two conductors in the PTC wire. In such an event, there occurs arcing or overheating at the specific areas in which the break occurred. It would be desirable, therefore, to provide some means in connection with a positive temperature coefficient heating wire blanket to interrupt the circuit to the blanket prior to there being a dangerous condition caused by the arcing of a broken conductor.
It is well-known in the electric blanket art to provide overheat protection means which include means to blow a fuse in the event of such an overheat condition. One such circuit is shown in the U.S. patent to Crowley, U.S. Pat. No. 3,628,093 in which a short circuit is created in connection with an overheat means and such short circuit is used to blow a protective fuse in the circuit. Another piece of prior art in which the safety circuit blows a fuse in connection with a malfunction in a blanket is the U.S. patent to Crowley, U.S. Pat. No. 4,034,185. The British Patent Specification No. 964,817, discloses several embodiments of an electric blanket having gas tubes connected across the heating elements to blow a fuse in the event of various types of shorts or opens in the blanket wire circuit. The blanket wire disclosed in the British Specification is broken into two separate heating elements and is quite different from the single positive temperature coefficient resistant heating element utilized with the present invention.
There are also many examples of protective circuits which include means for blowing the circuit fuse to protect the load in the event of an overvoltage condition. Examples of these patents are Muench, Jr. U.S. Pat. No. 3,600,634; Wilson U.S. Pat. No. 3,968,407; Voorhoeve U.S. Pat. No. 3,878,434; Hurtle U.S. Pat. No. 3,493,815 and Shattuck et al U.S. Pat. No. 3,215,896. Also of possible interest is the patent to McNulty U.S. Pat. No. 3,325,718 which senses a condition in a load and provides a circuit to overload and blow the circuit fuse to disconnect the load from the power supply. Also of interest relative to the specific circuitry used in such protective circuits is Lawson U.S. Pat. No. 3,845,355 which shows a photoresistor controlling an overload relay.